Color television tube

ABSTRACT

In a color television tube of the type including a striped tricolor luminescent screen and a postfocusing grid adjacent the screen, the grid is set at a potential higher than that which ensures focusing on the screen of the electron beam or beams impinging on the grid. Diaphragm means is provided to prevent lateral portions of the cross section of the beam or beams from impinging upon the screen. Secondary electron emissions are reduced or substantially eliminated because the electric field between screen and grid is reduced. As a result, the contrast provided by the tube is increased.

I limited States Patent lnventor Henri DeFrance 1, Rue dAujou, Asnieres,(Hauts de Seine), France Appl. No. 843,879 Filed May 22, 1969 PatentedFeb. 23, 1971 Priority Aug. 25, 1965 France 29357 Continuation-impart ofapplication Ser. No. 574,370, Aug. 23, 1966, now abandoned.

COLOR TELEVISION TUBE 5 Claims, 11 Drawing Figs.

U.S. C1 315/31, 313/83 Int. Cl H013 29/56, l-l01j 29/84 Field ofSearch313/83, 85,

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Malcolm F.l-lubler Attorney-Kurt Kelman ABSTRACT: In a color television tube ofthe type including a striped tricolor luminescent screen and apostfocusing grid adjacent the screen, the grid is set at a potentialhigher than that which ensures focusing on the screen of the electronbeam or beams impinging on the grid. Diaphragm means is provided toprevent lateral portions of the cross section of the beam or beams fromimpinging upon the screen. Secondary electron emissions are reduced orsubstantially eliminated because the electric field between screen andgrid is reduced. As a result, the contrast provided by the tube isincreased.

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HEM RI 4: Femce AQEAIT' COLOR TELEVISION TUBE REFERENCE TO RELATEDAPPLICATION This is a continuation-in-part of my copending applicationSer. No. 574,370, filed Aug. 23, 1966 and now abandoned.

The present invention relates to an improvement in color televisiontubes with a screen formed by phosphor strips, comprising, in theimmediate neighborhood of the screen and facing it, a grid of fine wiresheld at a potential lower than that of the screen and forming therewitha system of cylindrical lenses for each electron beam, or the singleelectron beam.

It is known that tubes of this kind present a clear advantage over tubeswhose screen is preceded by a shadow mask, that is to say, in the caseof a screen formed by phosphor strips, by a wall having only narrowapertures parallel to the phosphor strips, wherein the total surface ofthe apertures presents only a very small part of the total surface ofthe mask.

A grid tube has a higher optical efficiency and, therefore,

may reach a much greater brightness, since the majority of the electronsreaches the screen while the greater part of electrons is stopped by themask in a shadow mask tube.

However, the tubes with wire grids of known types have poor contrast,due to the secondary emission of the grid and the screen.

The present invention eliminates or at least considerably reduces thisdrawback by reducing the electric field in the space comprised betweenthe screen and the grid.

The present invention is applied to color television tubes of the typecomprising a screen formed by phosphor strips of different colors, and agrid facing the screen and adjacent thereto. The grid definesaperturesat least approximately parallel to the strips, and means is provided forproducing one or more electron beams directed toward the screen. A highpositive potential V is applied to the screen and a potential V, lowerthan V,; is applied to the grid whereby the gridscreen assembly definesa system of cylindrical lenses convergent for the beam or beams.According to this invention, the ratio of potential V, to potential V isgreater than thatwhich focuses the electron beam or beams on the screen,being greater than one-quarter of the potential V preferably betweenone-third and three-quarters of the potential V A diaphragm means ispreferably arranged to prevent lateral portions of the cross section ofthe electron beam from impinging upon the strips.

The above and other objects, advantages and features of the inventionwill become more apparent from the following detailed description ofcertain preferred embodiments, taken in conjunction with theaccompanying drawing wherein:

FIG. 1 diagrammatically shows a cross section of part of a known tube towhich the invention may be applied;

FIG. 2 shows the configuration of equipotential lines between the screenand the grid of the tube of FIG. 1, and the trajectories of the rays ofan electron beam in the space between the grid and screen; v

FIG. 3 illustrates the secondary emission phenomena inthe space betweengrid and screen;

FIG. 4 shows the purity defect which may be caused by an imperfectfocusing of the spot on the screen in the absence of a correction by acomplex diaphragm, limiting the width of the spot;

FIGS. 5 and 6 show an embodiment of the invention, using a complexdiaphragm acting at the level of the grid;

FIG. 7 shows a preferred embodiment of the complex diaphragm of FIGS. 5and 6;

FIGS. 8, 9, and 10 show three embodiments of the invention, usingcomplex diaphragms combined with the grid; and

FIG. 11 shows another embodiment of the invention, in which the complexdiaphragm is combined with the screen.

In all figures, the same reference letters or numerals designateidentical parts or elements. For the sake of clarity, the ratios betweendimensions are not drawn to scale.

The invention will be described, by way of example only, with referenceto a tube with a screen with vertical phosphor strips and with threeguns, the screen having a regular structure, i.e., formed by stripswhich are exactly parallel and of equal widths.

FIG. 1 shows diagrammatically in horizontal cross section the left partof a tube of this kind.

Inside the envelope 1 are shown:

The screen E formed by vertical phosphor strips, successively green, redand blue, three consecutive strips forming a triplet. The width of thesestrips, being about one-third of a millimeter, is shown greatly enlargedrelative to the dimensions of the envelope.

In front of the screen is a grid g of fine wires, parallel to the stripsand delimiting the triplets, as seen from a point lying on the axis ofthe tube, the spacing of the grid wires being constant and smaller thanthe width of a triplet. The distance between grid and screen may be ofthe order of 2 cm. and is also exaggerated in the drawing.

Only one of the three guns, i.e. the red gun, blue gun" and green gun,associated, respectively, with the phosphor strips of the three colors,is shown here, the gun being the center gun 3, in this case the red gun.

One of the elements 2 of the magnetic deflection unit is also shown.

Not shown are the conductive coating, often called the anode, coveringthe envelope internally between a vertical plane preceding the output ofthe guns and a vertical plane near the grid; the purity correctionelectrode, preferably use in these tubes; and the convergence device.

A-conventional mode of operation of this tube will now be brieflyoutlined.

The cathodes of the guns are approximately at ground potential.

The screen E is at a very high potential, of the order of 20,000 voltsor even more, while the grid 3 carries a much lower potential, forexample 5,000 V. The above-mentioned anode and the electrodes of thethree guns having the highest voltageare at voltages equal to that ofthe grid or at least of the same order.

The magnetic deflection unit sweeps the three beams coming from the gunsand intensity modulated by the red, blue and green signals, over thescreen, line by line, as in the case of monochrome tubes.

The deflection centers" of the red, green and blue beams, locatedin thesame vertical plane, the so-called deflection plane, are shown in FIG. 1by the straight line x-x'. The center of deflection of the red beam isindicated at 4, the beam itself being shown by a dotted line in'one ofits positions.

The grid-screen arrangement constitutes a system of cylindrical lensesfor the beams, which assures the focusing in the screen plane, i.e. theluminescent surface.

For a satisfactory picture, two conditions must be fulfilled, namelythose of purity and of convergence. The first requires that each beamshould at all times be focused on a strip of the associated color, thesecond that the three spots are not far from each other, i.e. correspondto the same point of the picture.

FIG. 2 shows on avery large scale the center portion of the screen withthe red, blue and green vertical strips R, G and B. These strips arecovered with a very thin conducting layer or coating 5, whereby thescreen potential can be applied to them, but sufficiently thin to allowthe electrons impinging on it to reach the phosphors forming the strips.

The drawing shows in broken lines the equipotential lines between screenand grid, and the beam 9 coming from the red gun, represented by anumberof electron trajectories drawn in solid lines. These electrontrajectories are substantially parallel and their common direction isnormal to the grid immediately in front of the space between grid andscreen. This perpendicular incidence follows from the fact that thecenter portion of the screen has been shown, but the parallelism of thetrajectories is an approximation which is valid for the whose surface ofthe screen, taking into account the magnitudes involved. In fact, thefocusing produced by the electron optical system of the gunreduces thediameter of the beam, which does not exceed about 3-4 mm. at the outletof the gun, to about 1 mm. immediately in front of the grid. Thedistance between the output of the gun and the grid is, for example,some 30-40 cm., and the trajectories may, therefore, be regarded assubstantially parallel for any sweep angle.

The grid-screen arrangement, with regard to the potential distributionin the space between grid and screen, forms an assembly of convergentcylindrical lenses (one lens corresponding to each grid interval) whichare substantially identical and have the same focal plane whichcoincides, as is known, substantially with the surface of the screen.

Each lens sequentially receives the beams coming from each of the threegu'ns'in the form of a beam of rays which are substantially'parallel andfocuses them at one point of its focal plane. If the condition of purityis observed, this focusing takes place for each beam on a phosphor stripof the associated color.

The focusing of the beams in the plane of the screen requires, inpractice, for a grid-screen spacing of the order of 2 cm., a ratior='V,/V equal approximately to l/4, where V is the screen potential andV, the grid potential.

This gives V V,=V r. V,; V,, (l r), with (l-i') of the order of 3/4.

It is necessary for V to be high, e.g., of the order of 20,000 -v. toassure the adequate excitation of the phosphors;

It'follows from the preceding that, in known tubes of this type, thepotential difference V V, is considerable, equal to 15,000 v., with V20,000 v. and r=l/4, which is, as will be shown, a serious drawback asfar as the secondary emission of the grid is concerned.

The quality of a tube with regard to contrast can be defined by theratio C L /L where L and L,, and respectively the maximum and minimumbrightnesses obtainable simultaneously on the screen with white light(by means of applying appropriate voltages to the three guns), with aspecific test pattern generator.

A high value of C is desirable, but grid secondary emission phenomenaprevent reaching the desired values in grid tubes of the known type.

- the space between grid and screen.

This drawing shows on a smaller scale the center portion of the screenthe corresponding part of the grid g, and the red beam 9.

The solid arrows 7 illustrate the initial parts of the trajectories ofsecondary electrons emitted by the coating due to the impact ofelectrons of the incident beam. Secondary electrons, reflected by thegrid, fall back onto different portions of the screen.

On the other hand, the incident beam liberates fromthe grid secondaryelectrons, many of which will reach the screen following trajectoriespractically perpendicular to it, such as illustrated by dotted lines 8.i

In accordance with the invention, an important reduction of unwantedsecondary electron emissions is obtained by using a ratio r =V,,/Vgreater than the value r =l/4, which ensures the focusing of the beambeyond the surface or plane of the screen.

However, the ratio r should not become too high and must remain smallerthan3/4. Otherwise, the focusing of the beam in the space between gridand screen becomes too weak for a correct operation of the tube. Thepreferred ratio is between H3 and 3/4.

Experience shows that it is possible to start, for instance, with ascreen voltage V of the order of 20,000 volts or more and a ratio r ofthe order of 0.3 to 0.35, and to adjust the barrier effect of theconductive coating so that a proper excitation of the screen by primaryelectron beams is obtained without any perceptible image of the grid andwith a defocusing sufficiently moderate so as not to involve anynoticeable purity defeet with a tube having phosphor strips of the orderof 0.3 millimeters wide, it being understood that the electron opticaldevices of the tube are matched to these conditions of operation.

Using a ratio r much greater than l/4, for instance r l/2, can, for agiven strip'width, lead to the undesired result that the diameter ofthe'spot on the screen becomes greater than the widthof the strips. Butthis defect can be corrected by means of a diaphragm in accordancewiththe invention.

One can use an independent complex diaphragm 10, as shown in FIG. 5,which shows the center portion of the screen E- and the grid g, and thebeam 9 whose shaded lateral parts are stopped by the complex diaphragm.The diaphragm may be, for example, spaced 1 cm. from the grid and havethe same potential as the latter.

. The complex diaphragm consists of vertical strips 11 which areimpermeable to electrons, separated by slots parallel to the strips soas to stop parts 90 of the beam 9 which, after 7 focusing or ratherpartial focusing by the grid-screen system, would produce lateralportions of the spot overlapping the red strip. Y

The strips 11 *are so arranged as to limit the beam in this manner atany angle of sweep, as shown in FIG. 6, which corresponds to FIG. 5 butshows another portion of the screen, where the incidence of the beam isno longer perpendicular to the grid. The dotted lines show the finaldiameter of the beam if the diaphragm were not present.

Generally, if the complex diaphragm restores the purity for the beam ofthe center gun, it will act satisfactorily for the other two beams too.However, the width of the strips maybe slightly adjusted in order toavoid any insufficiency.

It should be noted that, if the diaphragm l0 and the grid g an image onthe screen if the coating and the potentials used are not according tothe invention.

Consequently, the grid may be eliminated. However, it can be retained asan auxiliary element while being preferably brought to a voltage V',,slightlylower than the voltage V, applied to the diaphragm. It is easilyseen that this arrangement is advantageous as far as the effects ofsecondary emission are concerned.

Whether the grid g is retained or not, it should be noted that, sincethe beam incident on the diaphragm-grid 10.has a substantially circularcross section, the reduction in percentage of the surface of the beamcross section, due to the action of the diaphragm, is much less than thereduction in percentage of the width of the beam.

Moreover, since the density of the beam is greater in its center than atits periphery (the distribution of the electrons .is substantiallyGaussian along a diameter of the beam with its maximum on the axis), thepercent drop in the number of electrons reaching the screen is evensmaller.

This, however, is not an optimum solution, as it involves the.

risk ofhaloes appearing around the spots.

In the embodiment of FIG. 9, the effect of a diaphragm is producedineach grid interval by bounding this interval not by a single wire butby an assembly of three wires with coplanar axes and tangential crosssections. The diameters of the two lateral wires are preferably smallerthan that of the center wire, thus imparting excellent antivibrationproperties to the grid, since the respective resonant frequencies of thetwo outer wires and of the center wire are different. For example, thediameters may have a ratio of 7/8 although a higher ratio is shown inthe drawing. All three wires may have different diameters.

The diaphragm of FIG. 10 also uses three wires in a group, the wireshaving unequal diameters but their axes forming the edges of atriangular prism. The manufacturing of this arrangement is easier thanthat of the complex diaphragm of EH19.

Finally, FIG. 11 shows an embodiment of the invention, in which thediaphragm does not limit theiwidth of the beam but only the width of theactive portion of the beam.

Each luminescent strip of triplets is reduced in width and surrounded bytwo dead" strips 23 for which the screen support is not covered withphosphors and may be covered advantageously with carbon, for example.The width of the dead strips is such that they cover the portions of thescreen which the lateral portions of two beams overlap, and aresuperimposed so that the active portions of the three beams cannotoverlap, and each beam excites only the color band associated with it.

In FIG. 11 the width of the dead strips is more than the strict minimum(for which in the drawing the red beam would cover on the screen the redcenter strip R and the two adjacent dead strips), which facilitates theachievement of purity while allowing for larger manufacturing and/oradjustment tolerances. Naturally, reducing the width of the dead stripsto a minimum might be preferred, or on the contrary widening the deadstrips to enhance the advantage mentioned above with respect to purity,but the width of the luminescent strips must always remain sufiicient.

The invention, of course, may be applied to all color television tubeswith one or more guns, comprising a screen formed by phosphor stripsand, near the screen, at least one grid acting in conjunction with thescreen as a focusing means. When this grid is a color selection gridwith a variable potential, V, should be understood to mean its averagepotential.

I With tubes according to the invention, and a test pattern comprising ablack and a white portion respectively covering the left and the righthalf of the screen, contrast values have been obtained, which reached100 or more (against about 10 for a conventional tube) between twopoints located at a distance of 3 centimeters from the theoretical lineseparating the black and white portions, and a contrast of 250 or more(against approximately 20 for a conventional tube) between two pointslocated 10 centimeters from. said line.

lclaim:

l. A color television picture tube of the type comprising: a screenformed by phosphor strips of different colors; a grid facing the screenand adjacent thereto, said grid defining apertures substantiallyparallel to the strips; means for producing an electron beam directedtoward said screen; means for applying to the screen a high positivepotential V,,; means for applying to the grid a potential V, lower thanV,; whereby the grid-screen assembly defines a system of cylindricallenses convergent for said beam; and the potential V, being betweenone-third and three-quarters of the potential V 2. The color televisionpicture tube of claim 1, further comprising diaphragm means adapted toprevent lateral portions of the cross section of the electron beam fromimpinging upon said strips.

3. The color television picture tube of claim 2, wherein each gridinterval is bounded by two groups of three wires in contact with eachother. 7

4. The color television picture tube of claim 3, wherein the axes of thethree wires of each of said groups form the edges of a triangular prism.

5. The color television picture tube of claim 4, wherein at least two ofthe three wires of each group have different diameters.

1. A color television picture tube of the type comprising: a screenformed by phosphor strips of different colors; a grid facing the screenand adjacent thereto, said grid defining apertures substantiallyparallel to the strips; means for producing an electron beam directedtoward said screen; means for applying to the screen a high positivepotential VE; means for applying to the grid a potential Vg lower thanVE whereby the grid-screen assembly defines a system of cylindricallenses convergent for said beam; and the potential Vg being betweenonethird and three-quarters of the potential VE.
 2. The color televisionpicture tube of claim 1, further comprising diaphragm means adapted toprevent lateral portions of the cross section of the electron beam fromimpinging upon said strips.
 3. The color television picture tube ofclaim 2, wherein each grid interval is bounded by two groups of threewires in contact with each other.
 4. The color television picture tubeof claim 3, wherein the axes of the three wires of each of said groupsform the edges of a triangular prism.
 5. The color television picturetube of claim 4, wherein at least two of the three wires of each grouphave different diameters.